The molding of hydrophilic contact lenses is known. Various processes are disclosed in U.S. Pat. No. 4,495,313, to Larsen; U.S. Pat. No. 4,640,489 to Larsen, et al.; U.S. Pat. No. 4,680,336 to Larsen et al.; U.S. Pat. No. 4,889,664 to Larsen et al.; and U.S. Pat. No. 5,039,459 to Larsen et al., all of which are assigned to the assignee of the present invention.
These prior art and other references generally disclose a contact lens production process wherein each lens is molded from a reactive monomer or prepolymer mixture. The molding is done by a casting process in which the mixture to be polymerized is deposited into one first mold half, often referred to as a front curve, a second mold half, often referred to as a back curve is assembled onto the first mold half, and the assembled system is subjected to conditions resulting in polymerization of the mixture into a contact lens having the shape of the cavity formed between the two mold halves. These mold halves are usually formed from transparent thermoplastics such as polystyrene or polypropylene.
If the preassembly and assembly processes are carried out in an ambient environment, with the molds being exposed to air containing molecular oxygen (O.sub.2), the lenses produced sometimes are not of the desired quality. It is believed that this is due to the O.sub.2 coming into contact with the surface of and permeating into the plastic mold halves. It is believed that O.sub.2 on and in the plastic halves adversely affects the polymerization of the lens material. The effect of O.sub.2 on the photopolymerization process to strongly inhibit radical-induced polymerization is documented. Polymerization is suppressed until O.sub.2 has been consumed by reaction with radicals until the monomer is able to compete successfully with O.sub.2 for initiator radicals. Two types of systems have been identified: closed and open. Both types of systems apply to the present invention.
In the closed system, no O.sub.2 or a fixed amount of O.sub.2 is initially present in the system and polymerization proceeds appreciably after an induction period, during which the O.sub.2 is consumed by radicals. In the open system, O.sub.2 diffuses into the system and polymerization occurs only if sufficient radicals are generated to successfully compete with the O.sub.2. Open systems typically are systems that are open to air.
Exposing mold halves to O.sub.2 before assembly of the mold halves leads to a "closed-open" system during polymerization. O.sub.2 migrates into the mold by absorption creating an O.sub.2 reservoir. After the induction period when O.sub.2 in the monomer is consumed, polymerization proceeds in the lens bulk with no measurable effect from the O.sub.2 initially present. However, at the lens/optical mold surface interface (lens surface), some of the O.sub.2 absorbed into the mold now migrates back to that surface where it affects polymerization for a period extending beyond the induction period and causes the surface properties of the lens to differ from the bulk properties of the lens. The duration of this period and the extent to which it causes a measurable effect on lens properties is dependent on the amount of O.sub.2 absorbed into the mold prior to assembly when the system is "closed".
The effect of O.sub.2 absorbed onto or into the mold on photopolymerization of the reactive mixture is expected to disrupt polymerization at the lens surface, i.e. to cause differential polymerization at the lens surface relative to the lens bulk. This disruption causes more loose polymer ends at the surface due to (premature) termination of polymerization by O.sub.2. These shorter chain polymers at the surface of the lens tend to have lower cross link density, less chain entanglement, and more tackiness than the polymer chains in the bulk of the lens. These factors result in reduced mechanical strength and increased water content at the lens surface relative to these properties in the lens bulk.
Under oxygen-free molding conditions, lenses are isotropic in nature. As O.sub.2 is introduced to the lens surface and not to the lens bulk during polymerization, lenses become less isotropic in nature and more anisotropic, and control of final lens properties within specified tolerance ranges is compromised.
To reduce the deleterious effect of O.sub.2, contact lens manufacture has been carried out in a reduced O.sub.2 environment, and/or the reactive mixture is treated to remove dissolved O.sub.2 prior to polymerization. In manufacturing, this has resulted in the use of techniques such as physical enclosure of the process and use of large quantities of nitrogen to blanket the assembly and pre-assembly areas. This technique includes the plastic mold halves within the blanketed area since the boundary layer of gases on the plastic surfaces will include O.sub.2 if not so protected.
Various techniques for reducing the deleterious effects of O.sub.2 on the polymerization of contact lenses are found in the following U.S. Pat. Nos.:
5,362,767 Herbrechtmeier, et al 5,391,589 Kiguchi, et al 5,597,519 Martin, et al 5,656,210 Hill, et al 5,681,510 Valint, Jr., et al
EP Appln. No. 95937446.3 discloses a process in which plastic molds are treated prior to dosing with the reactive monomer mix to remove substantially all of the O.sub.2. The removal of the O.sub.2 can be accomplished by contacting the mold pieces with an inert gas or by using a vacuum. Molds that were not treated to remove the O.sub.2 provided contact lenses with high percentages of defects.
The use of an inert gas, such as N.sub.2 gives rise to a safety hazard since an inert gas requires elaborate sensing and alarming capability to protect personnel. Further, if the amount of inert gas surrounding the manufacturing equipment decreases for any reason, all the mold halves and lenses in that area of the line are discarded. Additionally, start-up after opening the inert gas enclosure requires time to "blow down", or reach an acceptable O.sub.2 level, before the product can be produced.
As would be expected, the use of N.sub.2, or other inert gas, adds cost and complexity of added equipment to the manufacturing process. It also adds time to the production cycle. Therefore, it would be desirable to be able to mold the lenses without the need of excess N.sub.2 or other inert gas.
By eliminating N.sub.2 or other inert gas from lens production, cost savings would be realized. Not only the cost of the gas, but also the cost of plumbing and control valves, compressed air plumbing and control valves, O.sub.2 process sensors, and inert gas safety sensors would be eliminated. The cost of calibrating and maintaining the process sensors and safety sensors would be eliminated. Lens production software control would be simplified with the elimination of inert gas plumbing, compressed air plumbing, process sensors, and control valves thereby providing a double benefit of not only initial development cost savings, but also operational cost savings and material savings. Also, elimination of the inert gas buffer would reduce equipment complexity and eliminate the associated work in progress problem, and would allow for further process simplification by minimizing the time from injection molding to assembly. Overall production line size would be significantly reduced.